Gyroscopes are sensors that measure the rate or angle of rotation. Micromachined gyroscopes, such as those based upon microelectromechanical systems (MEMS) have the potential to dominate the rate-sensor market mainly due to their small size, low power and low cost. At the same time these features means that the application domain for micromachined gyroscopes is quickly expanding from automotive to aerospace and consumer electronic industries, see for example Marek in “MEMS Technology—From Automotive to Consumer” (Proc. ASME Conf. on Microelectromechanical Systems, pp 59-60, 2007). Within the automotive sector multiple application exist including navigation, anti-skid, roll-over detection, next generation airbag, and anti-lock brake (ABS) systems. Micromachined gyroscopes can also be used for inertial navigation, namely the process of determining an object's position based on measurements provided by accelerometers and gyroscopes contained within an object or within a device associated with an object. An inertial measurement unit (IMU) typically uses three accelerometers and three gyroscopes oriented to gather information about an object's direction and heading. IMUs are vital components in aircraft, GPS-augmented navigation, and personal heading references, see for example Dixon et al in “Markets and Applications for MEMS Inertial Sensors” (Proc. IEEE MEMS/MOEMS Components and Their Applications III, Vol. 6113, pp 33-42, 2006).
In addition, there are numerous emerging consumer applications for micro-gyroscopes, including image stabilization in digital cameras, smart user-interfaces in handheld devices, gaming interfaces, and inertial pointing/location devices within these devices as well as smartphones, cellular telephones, PDAs etc. Further, small size, low power and low cost MEMS gyroscopes open new markets and applications such as integrated wireless/location tags for asset management and in areas like TV remote control applications, medical and industrial etc. As potential high volume consumer applications for micromachined gyroscopes continue to emerge, design and manufacturing techniques that improve their performance, shock survivability, and reliability without driving up the cost are becoming increasingly important. HIS iSuppli in March 2012 released market analysis indicating that MEMS gyroscopes in 2011 accounted for 41% of revenue for all kinds of motion sensors in consumer and mobile applications including accelerometers and electronic compasses, a market estimated at US$1.6 billion a rise from a 24% share in 2010 with an overall motion sensor revenue of approximately US$1.1 billion. In 2015 the MEMS gyroscope market is projected to reach approximately $1.1 billion alone (http://www.isuppli.com/MEMS-and-Sensors/MarketWatch/Pages/Gyroscopes-Are-Top-Earner-in-Consumer-and-Mobile-MEMS-for-2011.aspx).
Accordingly it would be beneficial against this market to provide solutions that further reduce the dimensions of the MEMS gyroscope, either directly or through removing requirements for ancillary control electronics. Such footprint reductions for discrete MEMS gyroscope die increase the number of die per wafer and hence reduce the unit cost given an essentially constant cost of fabricating a wafer with a high volume semiconductor manufacturing operation with comparable processing. In other scenarios wherein MEMS gyroscopes are integrated with control electronics and/or other electronics associated with the device within which the MEMS gyroscope is to be deployed then there is significant benefit from providing a manufacturing process for the MEMS gyroscope that is compatible with CMOS electronics.
CMOS electronics being the predominant technology for analog and digital integrated circuits in silicon due to the unparalleled benefits available from CMOS in the areas of circuit size, operating speed, energy efficiency and manufacturing costs which continue to improve from the geometric downsizing that comes with every new generation of semiconductor manufacturing processes. In respect of MEMS systems, CMOS is particularly suited as CMOS circuits dissipate power predominantly during operation and have very low static power consumption. This power consumption arising from the charging and discharging of various load capacitances within the CMOS circuits, mostly gate and wire capacitance, but also transistor drain and transistor source capacitances, whenever they are switched. Further, due to the relatively large dimensions of MEMS gyroscopes, typically hundreds of microns in diameter it would be beneficial for the MEMS gyroscope in some scenarios to be fabricated directly above the CMOS electronics rather than within a portion of the wafer adjacent to the CMOS electronics thereby reducing the die dimensions and lowering per die cost.
As MEMS gyroscopes are resonant devices requiring active excitation it would also beneficial to improve the resonator Q-factor reducing the electrical drive power requirements for the excitation circuitry. Similarly, as discussed below in the specification many MEMS gyroscope designs have multiple resonances arising from design and manufacturing considerations requiring the addition of frequency tuning and control circuitry together with the excitation/sense circuitry.
Accordingly, the inventors have addressed such issues by:                providing a novel design for bulk acoustic wave (BAW) MEMS gyroscopes that increases the vibration amplitude and provide increased sensitivity through electrostatic actuation combs;        providing novel BAW MEMS gyroscope designs with actuation electrodes within the central disc structure thereby increasing electrostatic drive and thus vibration amplitude and sensitivity;        providing BAW MEMS gyroscopes that can be fabricated directly above the CMOS electronics using low temperature BAW MEMS processing;        providing BAW MEMS fabrication processes allowing alternative materials to be employed with compatibility to low temperature processing;        providing compatibility to localized hermetic sealing methodologies for the MEMS gyroscope thereby easing packaging complexity of the MEMS-CMOS circuit;        providing novel BAW MEMS gyroscope designs eliminating release post-processing requirements;        providing BAW MEMS gyroscope designs that reduce the requirements for electronic frequency tuning; and        providing a BAW MEMS gyroscope with a novel side suspension scheme removing the requirement for the centre mounting of prior art BAW MEMS gyroscopes.        
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.